The following relates generally to the optical components used in optical communication networks, and specifically to a Micro-Electro-Mechanical Systems (MEMS) actuator that is used to rotate a discrete optical element, mounted to the MEMS actuator. More specifically, the following relates to the use of a MEMS actuator and rotating optical element to implement a variety of integrated optical components, providing functions that are useful in optical networks.
Optical components that switch, attenuate, filter, and process optical signals are widely deployed in optical networks, typically in the 1550 nm or 1310 nm wavelength windows. In many of these optical components, a rotating mirror is used as a beam-steering element. In these optical components that use rotating mirrors, Micro-Electrical-Mechanical System (MEMS) devices are often used to implement the rotating mirror or tilt-mirror element. In some prior art embodiments, the MEMS rotating mirror or tilt-mirror is fabricated from silicon using semiconductor processing methods and equipment. The mirror is fabricated as an integral part of the silicon device structure, and is often coated with a thin metallic layer (or layers), using gold, aluminum, or some combination of metal layers to achieve high reflectivity at the appropriate wavelengths.
FIG. 1 illustrates a representative prior art optical component that uses a rotating MEMS tilt-mirror as a beam-steering element, along with other fixed optical elements, excerpted from U.S. Pat. No. 6,838,738. Optical signals enter the optical component via one or more input fibers, 101 and 102. In this prior art embodiment, the input fibers and one or more output fibers, 103 and 104, are held in a single ferrule, 105, typically made of glass. The fiber ferrule 105 may have multiple bore holes, or a single bore hole that holds all of the input and output fibers. Light from the input fiber(s) then passes through a lens, 106, which is designed to form a collimated beam. The collimated beam may pass through one or more additional optical elements, represented generically in FIG. 1 by element 107. Note that depending on the nature of the function being performed by the optical component, optical element 107 might be an optical filter, or a diffraction grating, or some other form of passive, fixed optical element. In the case of a simple optical switch or attenuator, there might not be any need for optical element 107. The collimated beam then hits the reflective surface of a rotating MEMS tilt-mirror, 109, which is attached to, and rotated by, the MEMS device's actuator structure and chip framework, 108. It should be noted that in most embodiments of MEMS tilt-mirror devices, the rotating mirror 109 is an integral part of the MEMS device structure, and is made of the same material as the rest of the device, 108, including the portion of the MEMS device that serves as an actuator of the mirror rotation.
The MEMS device 108 will typically be mounted onto a chip header of some kind, shown as item 110, with electrical pins 111 that are used to carry control signals or voltages to the MEMS device, in order to control the rotating of the MEMS tilt-mirror 109. Some form of packaging or housing, 112, is used to position the lens 106 and other optical elements in the proper location and alignment, with respect to the MEMS device, and also to provide protection from environmental conditions.
Light that is reflected by the MEMS tilt-mirror 109 then passes back through the optional optical element 107, and is focused by the lens 106 onto the end-face of the fiber ferrule 105. Depending on the amount of tilt or rotation that is applied to the MEMS tilt-mirror, the light will be focused onto the core of one of the output fibers, 103 and 104, providing an optical switch function, or, alternatively, the focused beam may be only partially aimed at an output fiber, thereby achieving attenuation of the optical signal. The MEMS tilt-mirror may also be aimed such that there is minimal light coupled to any of the output fibers, creating a blocking, or OFF state.
FIGS. 2A and 2B show conceptual drawings of a prior art, single-axis MEMS tilt-mirror, to illustrate the basic principles of operation. In FIG. 2A, excerpted from U.S. Pat. No. 6,628,856, the rotating MEMS tilt-mirror 2A01 is suspended from an outer frame by torsion beams 2A02 and 2A03. These torsion beams define the rotational axis of the tilt-mirror 2A01, and also serve as torsion springs, applying a centering spring force that resists the tilting of the mirror 2A01. The rotational axis of the MEMS tilt-mirror is shown as item 2A04. In many MEMS tilt-mirrors, the device is fabricated from silicon, a crystalline material. Even though silicon is quite rigid, the torsion beams 2A02 and 2A03 can be designed to be thin enough and long enough to bend in response to an applied force. Typical range of motion for such a device can be as little as a fraction of a degree, or as large as 10 degrees, or even more, depending on the design of the device, as well as the applied voltage(s).
In many MEMS tilt-mirror devices, rotational force is applied in the form of electrostatic actuators, often using large numbers of interlaced comb fingers, and the application of precise control voltages to the comb fingers, as shown conceptually in FIG. 2B. Electrostatic actuation of MEMS devices is described in detail in U.S. Pat. No. 6,838,738, as well as in other prior art patents. However, the basic principle is represented in FIG. 2B. The rotating MEMS mirror 2B01 is suspended by torsion beams 2B02, which attach to a fixed outer frame, or torsion beam anchor points, 2B03. Comb fingers 2B07 are attached to the rotating part of the structure, whereas the interlaced comb fingers 2B04 are attached to the fixed part of the structure, 2B05 and 2B06. Note that the torsion beam anchor points 2B03, and the fixed comb finger areas 2B05 and 2B06, are all part of the fixed framework of the device. When a voltage difference is applied between or across the two sets of comb fingers, 2B07 and 2B04, a force is created that serves to rotate or tilt the tilt-mirror 2B01. The amount of rotation is a function of the design of the structure, the number and geometry of the comb fingers, the spring constant of the torsion beams, and the applied voltage. By precisely controlling the applied voltage, a precise tilt angle can be achieved, and maintained.
FIG. 3 shows a conceptual drawing of a two-axis MEMS tilt-mirror, excerpted from U.S. Pat. No. 6,628,856. The tilt-mirror 301 is suspended by torsion beams 302 and 303, from an intermediate “gimbal” structure or framework, 304. This gimbal structure is in turn suspended by torsion beams 305 and 306, from the MEMS device outer framework 307. Although not shown in FIG. 3, two sets of comb finger actuators are needed. One set is used to tilt the tilt-mirror 301 within the gimbal structure 304, and the other set of comb fingers is used to tilt the gimbal structure 304 within the outer framework 307. The two sets of comb finger actuators are electrically isolated from each other, so that the degree of tilt for the two rotational axes can be separately controlled. This sort of two-axis MEMS tilt-mirror can be used to steer the optical beam in three-dimensional space, along two axes of rotation.
As discussed earlier, it is often desirable to combine a MEMS tilt-mirror with other optical elements. FIG. 4 shows a conceptual view of a prior art tunable optical filter, in which a MEMS tilt-mirror is used to steer an optical beam that passes through a fixed-position diffraction grating, and is excerpted from U.S. Pat. No. 7,899,330. Sub-assembly 401 comprises a fiber ferrule that holds an input fiber and an output fiber, and a collimating lens. The optical signal on the input fiber consists of multiple wavelengths, each carrying its own information. The multi-wavelength collimated beam that emerges from sub-assembly 401 passes through a diffraction grating 402, which serves to disperse the multiple wavelengths, such that they exit the diffraction grating at slightly different angles. All of the dispersed wavelengths hit the surface of rotating MEMS tilt-mirror 404, which is mounted on a chip header or other structure 405. The precise tilt angle of the tilt-mirror 404 is controlled by the voltage control circuit 406. Based on the applied voltage, and the resulting tilt angle, only a small subset of the input wavelength range (for example, a single selected wavelength) will be aimed or directed properly, back through the diffraction grating 402, to sub-assembly 401 and the core of the output fiber. In FIG. 4, an additional optical element 403 is also shown. For example, a quarter-wave plate may be used to rotate the polarization of the light between the two passes of the diffraction grating, in order to reduce polarization-dependent loss (PDL) in the selected wavelength.
In the prior art embodiments described above, the moving part of the optical component has been a mirror, used to steer optical beams, or to change the incident angle of light onto other optical elements that are fixed in place. For some optical component designs, however, it is desirable or useful to be able to move other types of optical elements. For example, tunable optical filters may be implemented by physically moving or rotating thin-film optical filter chips, or diffraction gratings. FIG. 5 shows a prior art embodiment of a tunable optical filter that uses a rotating thin-film filter chip. The thin-film filter chip may be implementing a Fabry-Perot filter with a single resonant cavity. Or, it may be a multi-cavity optical filter, with a flatter passband. Multiple wavelengths on an input fiber, represented by λo, λi, and λn, pass through sub-assembly 501, comprising a single-fiber ferrule and a collimating lens, and then pass through a thin-film filter chip 502, that can be rotated via some mechanism, such as a small motor, or even a manual mechanism. A single selected wavelength, represented by λi, or a range of wavelengths, is passed to sub-assembly 503, comprising a focusing lens and a single-fiber ferrule, and is then coupled to the output fiber. The filter chip itself is designed to pass a single, fixed wavelength, or range of wavelengths, when the angle of the chip to the incident beam is 90 degrees. However, when the incident angle of the multi-wavelength beam is changed, by rotating the thin-film filter chip, the selected or tuned wavelength is changed or shifted, to a longer wavelength. Similarly, in the prior art embodiment shown in FIG. 4, instead of rotating the tilt-mirror 404, wavelength tuning could also be achieved by physically rotating the diffraction grating.